Realistic deep water shader
All you need is the Principled Volume shader.

Create a volumetric cube
Give the volume a very light color, and use Density to control it
Use real units for light intensity (lumens). Related answer: Simulating a 40W Light Bulb.

Lights: 10 million lumens (a really strong searchlight), Density: 0.1, Samples: 2000 + Denoiser